C-Type glasses have long been known in the glass fiber industry as being suitable as reinforcing fibers when chemical durability is needed. These type fibers are essentially alkali, alkaline earth, alumino borosilicate compositions with an early C-Type composition being exemplified by U.S. Pat. No. 2,308,857. K. L. Lowenstein, in the book entitled The Manufacturing Technology of Continuous Glass Fibers (Elsevier Scientific Publishing Co., 1973), at page 29, discloses an exemplary C-Type glass composition of 65% SiO.sub.2, 4% Al.sub.2 O.sub.3, 5% B.sub.2 O.sub.3, 3% MgO, 14% CaO, 8.5% Na.sub.2 O and 0.5% Fe.sub.2 O.sub.3. A more specific C-Type composition, which has been available for many years, is a composition of about 65.5% SiO.sub.2, about 3.8% Al.sub.2 O.sub.3, about 0.1% Fe.sub.2 O.sub.3, about 13.7% CaO, about 2.4% MgO, about 8.9% of (Na.sub.2 O+K.sub.2 O), about 0.2% TiO.sub.2, and about 5.5% B.sub.2 O.sub.3.
C glass compositions have desirable chemical durability, for example, durability with respect to water and sulfuric acid but, unfortunately, in forming such fibers, there is a propensity for the fibers to devitrify, that is, crystallize. The reason for this is that the so called "delta T", that is, the difference in temperature between (1) the temperature at which the glass has a viscosity (in poise) of log 2.5 and (2) the liquidus temperature, is low. As an example, the above-indicated commercially available C glass has a delta T of about 275.degree. F. with a liquidus temperature of about 2034.degree. F. and a log 2.5 viscosity at a temperature of about 2309.degree. F. Those skilled in the art will readily realize the inefficiencies that result when a stream of molten C glass is attenuated into fibers and such fibers devitrify.
In accordance with the present invention, an improved devitrification resistant, sulfuric acid resistant, and water resistant glass fiber is provided which fibers can be formed using conventional, continuous glass fiber forming technology without significant fear of devitrification. By employing the present formulations, improved forming efficiencies will be realized and, in some instances, the batch cost of these fibers will be less than previous conventional C-Type glass formulations. The present glasses will have a significantly higher delta T and their chemical durability will be at least as good as previous commercially available C glass fibers. Thus, in accordance with the present invention, there is provided a devitrification resistant, sulfuric acid resistant and water resistant glass fiber consisting essentially of, in weight percent, about 67% to about 70% SiO.sub.2, about 3% to about 4% Al.sub.2 O.sub.3, about 10% to about 13% CaO, about 2% to about 3% MgO, about 7% to about 10% of (Na.sub.2 O+K.sub.2 O), desirably about 7 to less than 10% of Na.sub.2 O, about 3% to about 6% B.sub.2 O.sub.3, and less than about 1% of each of Fe.sub.2 O.sub.3, TiO.sub.2 and F.sub.2, said glass fiber having a delta T of at least about 395.degree. F. and a durability (% fiber weight loss at 96.degree. C.) in water for 24 hours of less than about 1.6% and in 10% H.sub.2 SO.sub.4 for 24 hours of less than about 1.1%.